In aerospace systems, engine exhaust ducts, nose cones, firewalls, and reentry shield surfaces, often are exposed to high temperatures or large temperature gradients and must, accordingly, be insulated. Each application has unique problems which have rendered it difficult to provide an adequate thermal insulation that can be tailored for optimum performance in each application.
Recently, low-density ceramic fibers have been used for insulating aerospace surfaces. For example, the space shuttle's exterior surface is insulated with a plurality of ceramic tiles that are arranged in a closely spaced, ordered array. To provide the required fit, each tile is cut precisely from a fused ceramic blank. To form the blanks, silica fibers and other ceramic components were initially mixed into a slurry and cast into blocks. After drying, the blocks were sintered to form strong ceramic bonds between the overlapping fibers. The blocks were cut into smaller blanks that were subsequently milled into the final tiles. Each tile was bonded to an isolation pad with a high-temperature adhesive. The pad was, then, bonded to the underlying metallic substructure of the shuttle.
During takeoff and reentry, a differential surface temperature distribution exists over the surface of the space shuttle. The fused ceramic tiles are vulnerable to shear forces caused by the differential surface temperature distribution. To prevent breakage, each tile must be small (generally less than ten inches on a side) thereby creating enormous fabrication and assembly costs.
Glass coatings have been developed to improve thermal shock resistance for ceramics. U.S. Pat. No. 4,093,771 to Goldstein et al. discloses a borosilicate glass coating that is used on the surface of reusable silica insulation. U.S. Pat. No. 4,381,333 to Stewart et al. discloses a two-layer glass coating for silica insulation. The base layer has a high emittance and is preferably formed by combining a reactive borosilicate glass with an emittance agent, such as silicon tetraboride, silicon hexaboride, boron, or silicon carbide. The outer layer is formed from discrete, sintered glass particles to provide a high scattering coefficient. Preferably, fused silica or a reactive borosilicate glass having a higher silica content than the base layer is used for the outer layer. In either the Goldstein or Stewart patent, the coating is sprayed onto the underlying fiber insulation and is fired to form a glass.
Insulation may be formed with an unsolidified silica glass felt sandwiched between silica glass fiber cloth. The three layers are stitched together with silica glass thread (or another suitable refractory thread) and are bonded with adhesive to the surface to be protected. Similarly, a layering effect may be achieved by superposing a stitched blanket of silica and aluminoborosilicate fibers (commercially available under the trademark NEXTEL from 3M Company) over a separate, stitched blanket of silica fibers. By staggering the blankets and using suitable emittance coatings on the outer surfaces of the blankets, control of the insulative characteristics can be achieved, thereby countering the temperature distribution on and gradient through the insulation.
Lightweight fibrous insulation that permits a wide range of design choices in terms of insulative characteristics, strength, and durability is still needed, especially insulation that can withstand long exposures to high temperatures as are likely to occur for aerobraking orbital entries for lunar or Mars explorers.
U.S. Pat. No. 4,849,276 to Baker et al. describes a thermal insulation structure comprising a honeycomb core, fibrous ceramic insulation filling portions of the core and extending in a sheet over the core, and a reinforcing glass topcoat. Insulation of this type improves over the silica tiles, by permitting easier assembly of larger areas and active transpiration. Still, tiles of this type are inadequate for long duration exposure to the high temperatures likely to be incurred with aerobraking.
Ablative heat shields have been studied for a long while. These aerobrakes include materials that degrade and slough off when heated. The decomposition, degradation, and falling away of material (i.e. "ablation") is a primary means to dissipate the heat. As the ablation occurs, the aerosurface of the vehicle continues to change and the flight characteristics change as well. Furthermore, the best candidate ablative materials are relatively dense, introducing a severe mass penalty to the vehicle. Because the materials effectively burn off during reentry, the vehicles (or at least the tiles) must be expendable; they cannot be refurbished in an economically practical manner.
Boeing copending U.S. patent application Ser. No. 07/238,957 (incorporated by reference) describes a thermal protection system for a spacecraft that is patterned after a "water wall." Spaced electrodes are used to gel a sol gel within the pores of ceramic fiber insulation using direct current at low voltages and, thereby, to trap a fluid. Heat from the reentry may evaporate the fluid, but the sol can be restored by simply diffusing additional water into the structure. The evaporation provides transpirational cooling for the material. This thermal protection system concept also suffers a relatively severe mass penalty, however, from the high density of the carrier fluid and increased fabrication costs from the added structural complexity of spaced electrodes that are embedded within the fibrous ceramic.